System for subglottal pressure measurement and display during speech

ABSTRACT

A method and device is described for estimating the subglottal air pressure during speech or singing from the intraoral air pressure in essentially real-time by using a type of peak detection and extrapolation means that holds peaks in the low-pass filtered pressure signal for a period of time sufficient to allow their interpretation as real-time subglottal pressure. An electronic circuit suitable for implementing this function is described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of a provisional patent applicationhaving the same title filed by the present inventor in January of 2013.There is no serial number available at this time.

BACKGROUND Prior Art

The following is a tabulation of some prior art that presently appearsrelevant:

NONPATENT LITERATURE DOCUMENTS

-   Kitzing, P. & Löfqvist, A. Subglottal and oral air pressures during    phonation: Preliminary investigation using a miniature transducer    system. Medical and Biological Engineering 13: pp. 644-648 (1975).-   Baken, R. and Orlikoff, R., Clinical Measurement of Speech and    Voice, 2^(nd) Ed., Singular Publishing (2000).-   Rothenberg, M. A new inverse-filtering technique for deriving the    glottal air flow waveform during voicing J. of the Acoustic Society    of America 53, pp. 1632-1645 (1973)-   Hoffman M. R., Baggott, C. D., and Jiang J., Reliable Time to    Estimate Subglottal Pressure, J. of Voice. March; 23(2): pp. 169-174    (2009)

The primary energy source for the acoustic energy produced by the humanvoice is the air pressure in the lungs. This air pressure is sometimesreferred to as the tracheal pressure or the subglottal pressure, sincein the absence of a strong upper respiratory constriction these threepressures are approximately equal in speech or singing. However, becauseof the inaccessibility of the lungs, the air pressure in the lungsduring speech or singing is difficult to determine clinically. Mostprevious attempts to directly measure lung pressure during speech haveemployed highly invasive techniques that are not practical for routineclinical measurements or for speech training exercises, such as the useof a tracheal puncture, in which a hypodermic syringe connected to asystem for recording air pressure is inserted between the cartilaginousrings of the trachea (Hertegard, et al. (1995) Though useful forresearch purposes under appropriate medical supervision, this methodwould be rejected by most clinicians and patients for routine screeningor speech training exercises.

Another invasive technique that has been used for measuring subglottalpressure during speech consists of inserting a miniature pressuretransducer through the glottis into the trachea (Kitzing and Löfqvist1975). In this method, the vocal folds and nearby tissues must beanesthetized to suppress the glottal closure reflex that prevents thepotentially fatal aspiration of food or other foreign bodies. The needfor anesthetization and the potential complications from placing aforeign body into the subglottal space make this also generallyunacceptable for routine screening or speech training.

A non-invasive technique for measuring subglottal pressure during speechaccording to the previous art involves placing the subject in a hardwalled chamber called a plethysmograph, with an airtight seal at theneck. The variations in air pressure in the chamber reflect to someextent the compression of lung air, and a number of attempts have beenmade to deduce the variation of subglottal pressure from the chamberpressure. However, the plethysmograph method is problematic enough thatit is rarely used [Baken and Orlikoff, 2000]. That the plethysmographmethod has been used at all is testament to the importance ofnoninvasive measurement of lung pressure in understanding vocal functionin speech.

A fourth technique for measuring the subglottal pressure according tothe previous art involves interrupting the supraglottal airway duringthe voice production with a fast acting valve, for example a valvetermed a balloon valve, and recording the air pressure just behind thevalve. (Hoffman et al., 2009) According to the measurements of Hoffman,et al. (2009), after about 150 ms this pre-valve pressure has equalizedwith the subglottal pressure for a variety of voice modalities. However,the dependence of this method on the use by the subject of a voice modethat is not overly adducted as well as the acoustic distortion of thespeech caused by a mechanical valve assembly near the lips, make thismethod unacceptable for routine screening or speech training.

In 1973, the applicant introduced the concept of estimating subglottalpressure during speech by recording the peak intraoral air pressureduring unvoiced bilabial consonants (as /p/ in English) and using aninterpolation algorithm to estimate the lung pressure between theconsonants (M. Rothenberg, A new inverse-filtering technique forderiving the glottal air flow waveform during voicing Journal of theAcoustic Society of America, Vol. 53, 1632-1645). This method is basedon the fact that if the outlets of the oral chamber are closed forproducing the bilabial plosive (lips and velopharyngeal passage bothclosed), and the glottis is open for the articulation of the unvoicedconsonant, the intraoral pressure will equalize with the trachealpressure in a matter of milliseconds. We will refer to this method asthe interpolation technique.

However, the interpolation technique, as implemented according to theprevious art, does not provide a real-time measurement; it has been usedonly for the analysis of previously recorded speech or singing, using adigital computer to implement the interpolation algorithm. One result ofthis limitation is that the interpolation technique as implementedaccording to the previous art could not be used for biofeedback in voicetraining exercises. In addition, the technique implemented according tothe previous art is cumbersome when used in routine speech testing.

The present application is for a method and device that will apply aversion of the interpolation technique for subglottal pressureestimation from intraoral air pressure during speech or singing inessentially real-time by using real-time low-pass filtering and apeak-hold system that holds a measured peak value for a period of timesufficient for observation.

SUMMARY

It an object of this invention to extend the interpolation technique forestimating subglottal pressure during speech from the intraoral airpressure, as described by the inventor in Rothenberg 1973, to makingreal-time measurements of subglottal pressure suitable for use inmedical screening and in speech training exercises. This is accomplishedby using real-time low-pass filtering of the intraoral pressure signaland using a peak detection and extrapolation means that holds thefiltered pressure signal peak for a period of time sufficient to allowits observation.

DRAWINGS Figures

FIG. 1 Block diagram of a system for subglottal measurement and displayduring speech.

FIG. 2 Electrical circuit diagram for a standard peak-hold circuit.

FIG. 3 Chart showing the time response of a standard peak-hold circuitto a negative step in the input of magnitude one, going from one volt tozero volts at t=0.

FIG. 4 An electrical circuit diagram for an augmented peak-hold circuitdescribed in this application having two stages, with a time constant ofone second in each stage.

FIG. 5 Chart showing the response at Vout of the circuit in FIG. 4 to anegative step in the input of magnitude one, going from one volt to zerovolts at t=0.

FIG. 6 An electrical circuit diagram for an augmented peak-hold circuitdescribed in this application having three stages, with a time constantof one second in each stage.

FIG. 7 Chart showing the response at Vout of the circuit in FIG. 6 to anegative step in the input of magnitude one, going from one volt to zerovolts at t=0.

FIG. 8 Chart showing the responses to a negative step in the input ofmagnitude one at t=0, going from one volt to zero volts, at Vout of anaugmented peak-hold circuit such as that in FIG. 6 but having M stages,with M going from 1 to 10, with the time constant in each stage equal to1/M.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the method and device described in this application, a tube 1 with asmall diameter, nominally a few millimeters, is inserted into the oralchamber of the subject's vocal tract, nominally through the lips at thecorner of the mouth, so that the tube senses the intraoral air pressurewhen the lips are closed.

The sensing tube is connected to a pressure transducer and preamplifier2 having a range commensurate with the lung pressures being measured.The output of the transducer and preamplifier is input to a low-passfilter 3 that essentially removes the speech acoustic energy, that is,the energy above about 40 Hz.

The output of the low pass filter is input to a system 4 for detectingthe peak value during each intraoral pressure pulse that occurs duringan unvoiced bilabial stop consonant. The system 4 must be designed toalso hold the last peak value for a period long enough to enable areading of the value of the peak and decay thereafter in a matter ofseconds.

From experiments with natural speech, performed by the inventor, a holdtime of approximately 0.5 second is suggested. In the absence ofadditional intraoral pressure peaks after the hold period, in order toallow future pressure peaks to be accepted if they are lower than theprevious peak, the output of peak detection and hold system 4 shoulddecay at a rate such that the output decays to at least half its peakvalue in not more than a second. We will call this the terminal decay.This degree of terminal decay would be accomplished by an average rateof terminal decay, after the hold time, of approximately 5% in 0.1seconds.

Finally, the output of peak detection and hold system 4 is sent to adisplay or recording means 5, which may be an analog meter, a digitalmeter, or an array of LED lights. The indicating means in 5 may beaugmented by a recording means to preserve the output of the peakdetection and hold system 4 for future reference.

The peak detection and hold system 4 could be implemented by a suitablyprogrammed microprocessor; however, a suitable analog electrical circuitdevised by the inventor for this purpose is described below. It isenvisioned that the analog circuit described in this application wouldbe more economical to produce than a microprocessor version, and so keepdown the cost of the device. On the other hand, a microprocessor versionof the peak detection and hold system 4 could be programmed to detectand measure a number more than one of such peaks and perform morecomplex operations involving such peaks, such as averaging orinterpolation.

Derivation of the Analog Peak Detection and Extrapolation Circuit

Though there are many forms of peak-hold analog circuits disclosed inthe literature, perhaps the simplest and most basic simple peak-holdcircuit is shown in the FIG. 2 with some typical circuit values. Thediode D1 charges the capacitor C1 to a local peak of Vin, providing thecapacitor voltage is not already greater than such peak.

C1 then discharges through the ‘drain resistor’ R, with the voltage onC1 decaying exponentially. Assuming a perfect op-amp and diode, thedecay in capacitance voltage is exponential with time constant R1×C1, aslong as the input to the diode Vin is never larger than the capacitancevoltage. The output voltage resulting from an input peak of 1.0 followedby a sharp decay to zero (a negative step function going from 1 to 0) isshown in FIG. 3 for two values of the time constant R1×C1. The value of10 for the longer time constant was chosen as the approximate minimumrequired to meet our 5% decay criterion for hold time. Note, however,that if the time constant is chosen to meet the 5% criterion, i.e.,R1×C1 is equal to or greater than 10 seconds, then the terminal decaytime may be too great. More precisely, a circuit with a time constant of10 seconds will take 10 seconds to decay to 1/e=0.37).

To solve the problem of a conflict between the desired hold time and thedesired terminal decay time, this application presents a modification ofa standard peak-hold circuit in which the drain resistor is returned tovoltage input to the diode instead of to ground, as are R1 and R2 inFIG. 4.

We will refer to such a circuit as an augmented peak-hold circuit, orAP-H. Using an augmented peak-hold circuit slows the initial dischargeof the capacitor C2, thus creating a period in which the output is heldnear the peak value.

Let us assume that a hold in the decay is marked by a decay of less than5%. In FIG. 5 it can be seen that by adding an AP-H stage with the sametime constant as the first stage, the decay period is increased fromapproximately 0.1 s to 0.4 s, a fourfold increase, while increasing theterminal decay time by less than a factor of two.

It should be noted that cascading two stages of the standard peak-holdcircuit in FIG. 2 would have no such effect, though a roughly similareffect may be obtained by adding an inductance in series with R1 of thestandard sample hold circuit.

The time that the peak voltage is held (stays within approximately 95%of the initial peak) can be further increased by using an additionalstage of AP-H, as in FIG. 6.

The operation of the circuit in FIG. 6 is illustrated in FIG. 7. Thecurves in the chart of FIG. 7 show the voltage at the outputs of thethree stages, referred to as V1, V2 and V3 respectively, after the inputVin leaves its peak value and goes quickly to zero (a negative stepfunction).

FIG. 7 shows that with three AP-H sections, and with all sections havingthe same unity time constant, the output is still at over 95% of thepeak value after 0.8 seconds (rounded to one significant figure). Thisis a reasonable value for real-time observation; however, the extensionof the hold time can be further increased with more sections cascaded.In fact, it can be shown mathematically that the response of M cascadedsections, all having a time constant of one second, to an input with avalue of one unit that drops quickly to zero, is given by the followingexpression in Equation 1, where equation 1 represents the response of afilter having M stages with the time constant for each respective stageequal to 1, to a negative step of unity amplitude (voltage going from 1to 0 at=0).

$\begin{matrix}{^{- t}{\sum\limits_{K = 1}^{M}\frac{t^{({M - K})}}{\left( {M - K} \right)!}}} & {{Equation}\mspace{14mu} 1}\end{matrix}$

As can be seen in FIG. 7, as cascaded stages are added having the sametime constant, the delay caused by the filter increases approximately inproportion to the number of stages.

This undesirable result of adding cascaded stages can be compensated forby using a time constant that varies inversely with the number of stagesin the filter. For example, if the time constant for a single stagefilter is assumed to be one second, then the time constant for eachstage of an M-stage filter would then be 1/M. The response expression inEquation 1 then would be as follows in Equation 2, where Equation 2represents the response of an M-stage filter to a negative step of unityamplitude (voltage going from 1 to 0 at t=0) for M stages, with the timeconstant for each respective stage equal to 1/M.

$\begin{matrix}{^{{- M}\; t}{\sum\limits_{K = 1}^{M}\frac{({Mt})^{({M - K})}}{\left( {M - K} \right)!}}} & {{Equation}\mspace{14mu} 2}\end{matrix}$

In FIG. 8, the responses of the filters in Equation 2 are plotted for Mequal one to ten. We will look more closely at the case of M=5, as apractical compromise between complexity and performance.

FIG. 8 shows that according to our 95% decay criterion, a 5-stagecascaded AP-H circuit, with time constants equal to 1/M=⅕ second wouldhave a delay of approximately 0.4 seconds, while having a final decay toa third of its peak value in approximately 1.1 seconds. The maximumdecay rate can be estimated from the graph to be approximately 9.5% in0.1 second, and the average decay rate after the hold period is roughlyhalf of that. These figures show that 4 or 5-stage APH circuit would fitour experimentally determined criteria, and this conclusion has beenconfirmed by tests using a 4-stage APH circuit, though the optimalnumber of stages and the optimal values of the RC time constants in eachstage should be determined by further testing.

The method and system may, of course, may be carried out in specificways other than those set forth without departing from the spirit andessential characteristics of the invention. Therefore the presentedembodiments should be considered in all respects as illustrative and notrestrictive, and all modifications falling within the meaning andequivalency range of the appended claims are intended to be embracedtherein. For example, there are many possible implementations of astandard peak-hold circuit, each of which could be converted to anaugmented peak-hold circuit with a plurality of stages for the purposedescribed in this application. The scope of this patent should not belimited to the example given for a standard peak-hold circuit.

What is claimed is:
 1. An apparatus for indicating the air pressure inthe lungs during speech or singing comprising: a pressure transducer forconverting air pressure to an electrical signal; a sensing tube forpassing the intraoral pressure to said pressure transducer; a low passfilter for receiving said electrical signal from said pressuretransducer and removing the acoustic energy from the signal; a peakdetection and holding means for receiving the output of said low passfilter and detecting the peak values of the output of the low passfilter and holding said values, or an approximation of said values, fora period of time; a recording or display means for displaying thepresent value of the output of said peak detection and holding means. 2.An apparatus according to claim 1 in which the peak detection andholding means is a peak-hold electronic circuit having a plurality ofstages in which in at least one of the stages the drain resistance isreferred to the input of the stage instead of to ground.
 3. An apparatusaccording to claim 1 in which the peak detection and holding means is amicroprocessor programmed to perform the hold and decay functions.
 4. Anapparatus according to claim 3 in which the microprocessor comprisingthe peak detection and holding means is programmed to record pluralityof successive peaks and compute an average of, or interpolation between,the values of such peaks.
 5. An apparatus according to claim 1 in whichthe peak detection and hold means is contains a peak-hold electroniccircuit in which an inductance is in series with a drain resistor inorder to effect a hold period.